Truth be told I didn't really bother with the rank system myself. I just figured I'd work up a scenario and see how you all handle it. Admittedly, I do tailor some of my missions to not only ensure that those involved can do something which allows them to shine, but also situations were they have to think outside the box (Of their abilities) if they want a good end. 

Additionally I also see the ranks as what the administrators of Beacon think the difficulty is, not necessarily what actually occurs. An example if y'all would remember would be way back in the beginning of Naruto where team 7 has to escort that bridge builder. I think that was a D or C rank mission? However things turned out to be really complicated and elevated things to at least an A rank mission. Similar instance. It's not that all my low rank missions are actually really tough ones in disguise, but if that's how the wind blows...